I tether from my iPhone. The iPhone isn't cheap, and neither are the phone plans to go along with it. My plan allows me to tether at no extra charge. I'm already paying these premiums; maximizing it with the seamless tethering between iOS devices is just being cost-effective. Why pay for data for an iPad and pay the premium for the cellular model when I've already paid for a high-end phone and its own data?
This is what I have been thinking. Also, by tethering to my iPhone I can regulate how much I use it and will keep me from using too much data. I really wanted a cellular iPad but I really don't want to pay for it.